Measurements of the spatial spreading of light in typical first-year sea ice in McMurdo Sound, Antarctica, have been characterized in terms of depth dependent scattering using Monte Carlo simulations. The results are shown to place constraints on models of the optical transmission of sea ice and on the relationship of the optical scattering parameters to the physical structure of the ice. Comparisons for different sites and seasons within a given year and between years are presented. The backscattering spatial profiles are shown to fix the scattering parameters of the strongly scattering top layer and the horizontal component of the anisotropic scattering parameter in the bulk layer. The transmission spatial profiles determine the anisotropy of the bulk layer and the absorption by algae near the base of the ice. It is demonstrated that the top layer is very sensitive to warming of the ice and that the scattering properties of this layer largely determine the albedo of sea ice while the transmission is affected by the nature of the top layer, the scattering in the bulk layer, and the absorption in the algal layer. A quantitative relationship between the scattering length and the air volume fraction is demonstrated.¿ 1997 American Geophysical Union